This invention relates to the contact printing of an original, such as a photographic negative, on a light-sensitive material by exposure. The invention deals more specifically with an improved contact printing method and apparatus employing vacuum to hold the original and the light-sensitive material in good contact with one another during exposure.
The vacuum contact printing method had gained extensive acceptance for accurate reproduction of the original image in photoduplication and the photographic preparation of printing plates, among other applications. This known method employs what is known as a vacuum printing frame, having a vacuum chamber between a rigid, light-transmitting plate, normally a pane of glass, and an elastic diaphragm of rubber or like material. The vacuum chamber houses an original (e.g., a photographic negative) and a light-sensitive material (e.g., a sensitive film or presensitized (PS) plate) in superposition. Upon pumping the vacuum chamber to a desired degree of vacuum, the difference between this vacuum and the atmospheric pressure acts on the diaphragm, causing the same to press the superposed original and light-sensitive material against the glass pane. Thus brought into contact with each other, the original and the light-sensitive material are then exposed through the glass pane.
This vacuum printing method, however, works well only in cases where either the original or the light-sensitive material is of paper, cloth, or like substances permeable to air. A problem arises when both are of nonpermeable substances. For example, if the light-sensitive material is a large, smooth-surfaced photographic film or PS plate for printing, and if the original is a similarly large film, then in the conventional vacuum printing frame air pockets will be readily produced between the original and the light-sensitive material and between the original and the glass pane. Such air pockets, particularly pronounced at the central regions of the superposed members, are readily recognizable from interference fringes known as Newton's rings.
The reason for the production of such air pockets is that upon evacuation of the vacuum chamber, the original and the light-sensitive materal have their peripheral portions first urged into close contact with the adjacent surfaces, thereby entrapping air between themselves and the adjacent surfaces. The removal of the entrapped air necessitates the evacuation of the vacuum chamber to a greater degree, until finally the air is forced out of the edges of the contacting members. An evacuation time as long as several minutes, or even several tens of minutes, has been common for the establishment of uniform contact between nonpermeable, flat-surfaced materials.
A conventional measure taken to circumvent this problem has been to roughen the contact surfaces of the original and the light-sensitive material to such a degree as to give no adverse effect on exposure. When brought into contact with each other, the roughened contact surfacs create a multitude of minute space therebetween, permitting ready withdrawal of air that might otherwise be confined between the contact surfaces. This known measure makes possible the establishment of uniform contact between the original and the light-sensitive material twice or even several times as fast as in the case where they are smooth-surfaced, although the evacuation time depends greatly upon the degree of roughening of the contact surfaces. As far as the applicant is aware, this has so far been just about the only method of obtaining good contact in a comparatively short time.